Teleporters. (They'd come as a set of two.) Using nether portals is okay but I'd love a single block platform you could step on to and have it teleport you to its partner in other parts of your world. You could set up homes away from home or a make a fast travel back to a stronghold or mineshaft! Just imagine a cool room in your base lined up with teleporters all signposted! :D

What’s your greatest Minecraftin’ achievement?

Probably firing an arrow a reeeeeally long distance in the nether. It hit a ghast's fireball and deflected it back into the ghast, killing it. It was half-luck, half-skill!